Process for the production of magnesium



Dec. 18, 1945. J. C'. RADEMAKER 2,391,193

PROCESS FOR THE PRODUCTION OF MGNESIUM Filed Aug. 4, 1944 INVENToR. ./oH/v C X940 MAKER Patented Dec. 1,945

rnocnss FOETH' PRODUCTION OF MAGNESIUM John C. Rademaker, Palo Alto, Calif., assignor, by mesne assignments, to The Anglo California National Bank of -San Francisco, San Francisco, Calif., a nationalbanking association, as trasf-4 tee 'for the benefit of Reconstruction Finance Corporation Application August 4,1944, SerlalNo. 548,027

(Cl. 'Z5-67) v zciaims.

This inventionrelates to the production of metallic magnesium by the reduction oi' magnesium oxide-containing raw materials, particularly of calcined dolomite or magnesite, with the aid of a reducing agent such as silicon, fer rosilicon, the silicides of calcium and magnesium, aluminum, or mixtures thereof, or of similar reducing agents which do not generate gaseous products of oxidation and is a continuation-inpart of my co-pending application Serial Number 492,465 filed June 26, 1943.

It is well known that in effecting the above reduction operation with the, use of a continuously fed furnace various diiiiculties are eni countered as well in the ytype ln-which the operation is eii'ected at a temperature above the melting point of the reaction components, as in the other type in which the mixture .is heated at a temperature 'below its melting point. Both processes are disadvantageous.` The reduction of magnesia in the liquid state 'is a rather slowA reaction leaving behind a considerable proportion of the unaltered magnesia; moreover disensagement of the magnesium vapor from the molten metal mass is difficult;v finally there is an appreciable attack on the furnace lining by the moltenslag. On the other hand reduction exclusively inthe solid state, while proceeding at a more rapid rate, vgives `rise to difficulties in conducting. the operation such as to obtain economical rates of vapor ow, the vapor evolved will carry over dusty impurities into the condenser, as a consequence of which the recovered metal will not answer to practical purity requirements.

'I'he art has therefore resorted to batch or intermittent processes. However, such operations have generally been disadvantageous for the reason that relatively small quantities of metal are 'recovered in the course of a single operation and because it has :been considered useful to preheat the v.charge containing themixture of magnesium oxide-containingraw material and the' reducing -agent. Furthermore, another great disadvantage of the prior art is that it has always been thought necessary to delay discharging the retort until it cools to a temperature below 80- lfi C. since otherwise the magnesium'would react with the atmospheric oxygen and nitrogen.

' which reaction may be so violent that the magnesium maybe set on fire.

It lathe-,object ofthe invention to enable the magnesilunvapor to be disengaged 4at a rate hi'gh enough to obtain commercially economical rates of vapor flow. It is: also an object of the invention to recover metallic magnesium in a very compact deposit, practically free from solids carried over by the vapor. Another object of the ,of Figure 1.

invention is to reduce the time required for carrying out the whole cycle of operations to a minimum and tol thereby enable the operation to be carried out on' a large scale. A further object of the invention is to provide apparatus for carrying out the process with efllciency and economy, and this application is directed to such apparatus. f

Other objects and advantages will be apparent from the following specification in connection with the accompanying drawing which shows one' formi of apparatus for carrying out the invention.

Referring to the drawing: K Figure 1 is a' horizontal elevation in section through the apparatus embodying my invention;

Figure 2 is a section taken on the line 2-2 Referring to Figures 1 and 2, a furnace F is shown which advantageously is provided with an outer shell or casing I having a refractorylining 2l which serves as the insulating medium for the walls and the flat top roof 3. The furnace roof has a, plurality of outlets 4 leading to stack 5 which permit any products of combustion pass from the furnace. A series of retorts 6, open at one end and closed at the other, preferably of the metallic type, are

horizontally disposed within the furnace in parallel relation to each other and in such manner that the end section of each retort extends outside thefront wall of the 'furnace and forms a condenser section of a size sumcient to collect the crystalline .metallic magnesium produced in a single cycle. A heat dam or reflector 8 is inserted within each retort in a position which separates the condenser section from that section of the retort exposed to the furnace. A'dished lid or closure 9 is provided to seal each of the retorts 8 in an air-tight manner. Vacuum pipes IU extend through the condenser section I 'into each retort. 'Ihe height of the retorts B within the furnace should be suicient to permit ready access to them either manually or by mechanical means, and furthermore, they are supported at such' height by iirebrick piers Il which are corbeled or wl'denedout in their mid-section (see Figure 1).

The vfurnace may advantageously be heated by natural gas which is supplied through a series of manifolds l2 and pipes I 3, which lead tothe .burners I4. Theburners I4 have one or `more rebrick fiamespreaders I5 xed at their tops and are located in series immediately adjacent the corbeled mid-sections of the piers slightly below Athe retorts and in parallel relation thereto.

eandescent zone. Itis to be noted' that the'briek's V supporting the retorts 8 at the top of `the piers are preferably of the chrome type so that there will be no reaction with the retort itself.

It is'to be understood that any type of heat darn may be employed in carrying out the invention so long as it satisfactorily separates the heated section of the retort containing the retort charge from the condensing sectionl and at the same time permits the passage of the evolved magnesium vapors.

In operation lthe furnace and retorts are heated to normal reduction temperature. that is, to a temperature within the range of 1,150

to 1260 C. and the retorts are charged with compacted mixtures of magnesium oxide-containing raw material and a reducing agent. Thereafter. the heat dam 8 is inserted within the retort in its proper position and the vacuumtlght lid 9 is inserted and the retort .is sealed. f

A relatively low vacuum is applied to the retort through suitable pumps leading to pipe I; This vacuum is advantageously Within the range of from 10 to 150 millimeters. absolute pressure. or at least' suificientlv low vto prevent any magnesium from being distilled in the condenser section of the retort. The purpose of this low sity of preheating the furnace charge by removing concomitants such as water vapor, carbon dioxide. air, and dust. The low vacuum is maintained for a period sumciently long to achieve the above end.

After the low vacuum period is over, additional vacuum pumps are put on the line leading to connection l0 and the vacuum within the retort is increased -to apoint where the reaction between the reducing agentl and raw force and form a condensed deposit of crystalline magnesium that is very solid and compact.

After the available Jmagnesium contained in the retort charge is completely distilled and recovered as crystalline metallic magnesium in the .condensersection the retort may be opened.

Any sodium evolved during the course of the reaction may be collected inta suitable collector (not shown) which, lif used, is advantageously located near the outer lend of the condenser vsection and can be readily removed withoutk ignitingthe magnesium. There is no reaction when vacuum step is primarily to eliminate the neeesthe retort is opened and the magnesium is oxposed to the oxygen and nitrogen of the atmosphere.

It is to be understood that the above cycle I may be repeated without lowering the furnace temperaturev so `long as"l the spent residue remaining in the retort is discharged. In fact the expedient of maintaining the retorts at substantially constant temperature throughout the ventire cycle is advantageous because it prolonge the life of the retorts by preventing the occurrence of heat shocks and thermal stresses therein which'normallyfoccur upon substantial variations in temperature. Consequently, as soon as the retort residue from one cycle is discharged the retort may be recharged, sealed and the vacuum applied inthe new cycle as previously described.

It hasbeen found advantageous touse a quanslightly in excess of that necessary to reduce the available magnesium contained in the inagnesium oxide-containing raw material. It is useful to recover the magnesium in large masses of solid compact form as distinguished from small pieces because the larger masses may be melted with less melting loss during the subsequent melting step. This mannerl of avoiding melting losses represents considerable saving in large scale operations.- f It is desirable to prepare the feed for the retort charge in a very compact-form comprising a mixfor the reason that the presence of dust decreases the purity of the magnesium metal recovered and cuts down heating efficiency within the retort The compacted materialmay be in the shape of nodules,.pellet s, briquettes or the like, which may be of any suitable size s0 long as they do not present a dust problem.

In preparing the retort charge it has been found advisable to grind the'materials before the compacting step. The desirable particle size for the raw material-'containing magnesium oxide, for example dolomite, maybe within the range of from 60 to 90 percent minus V200 mesh with ver? few particles coarser than minus mesh. The particle size of thev reducing agent, for example ferro-silicon, may be within the range of 50 to 'I5l 0 percent minus 200mesh with 10 mesh.

T'he following is an example of a ner of carrying out my invention:

Briquettes of a volume of roughly about i cubic inch and comprising 4.5 parts of dolomite ground to about 75 percent minus 200 mesh and 1 part ferro-silicon ground to about 85 percent minus ltraces up to minus typical mando 200 mesh are charged to a retort-heated to a temperature of about 1165`C. The retort is sealed and a-vacuum of about millimeters, absolute pressure, is applied for a period of about one and one-half vhours to remove all concomitants from 05 the charge and the retort itself. Thereafter the vacuum is increased to about 50 micronsfabsolute pressure, and magnesium vapor is evolved from the retort charge and condensed in the cool section. The high vacuum' is maintained for a period of about six and one-half hours and immediately thereafter the vacuum is broken with air and the retort opened. Under the'conditlons of temperature, pressure, and ratio -of feed/prescribed above, it is found that the available magnesiumpresent in the retort charge is' completely tity of reducing agent in the furnace chargev ture of the raw material and the reducing agent f It is to be understood that any type of retort may be employed in carrying out my invention so long as it is capable of withstanding the condii tions to `which it is subjected during the course of the process. Retorts with a single open end. or with each end open and extending beyond the furnace walls may be utilized. The retorts may be made oi' any type of heat resistant material such as high temperature resistant metals and refractories. A particularly useful type of retort which has been found to be of considerable value from the standpoint of economics and ease of manufacture comprises a retort of mild steel having an outer coating of alloy steel. The mild steel base material may vary in thickness from 1% to 2 inch and the. coating of alloy steel may A at normal reduction temperatures for a period vary in thiomess from 1/8 to inch. The inside may advantageously be alloy steel of the type having a chromium content oi 26 to 30 parts and a nickel content of 14 to 20. The higher chrome content is preferable.

1. In the production oi metallic magnesium by reduction of magnesium oxide-containing raw material with the aid of reducing agents generating non-gaseous products of oxidation, the method which comprises forming a reaction mixture comprising the initial material and a reducing agent, heating said mixture under vacuum suicient to completely remove undesirable concomitants without distilling magnesium, thereafter increasing said vacuum while maintaining constant temperature to cause magnesium vapor to be given oil, and cooling said vapor to a temperature below the solidification point of mag-I nesium. l

2. The process of claim agent is errosilicon.

f JOHN C.

1 wherein the reducing 

